Meiosis — Free AP Biology Lesson

The idea, then worked examples solved step by step, then practice questions whose numbers change every time you try them.

The idea

Meiosis: halving the set so a new generation can begin

Mitosis copies a nucleus exactly; meiosis has a different job: it halves the chromosome sets and produces genetically different nuclei. Meiosis is the nuclear division that forms haploid nuclei from a diploid starting nucleus. A haploid cell has one set of chromosomes, and a diploid cell has two sets of chromosomes; the ploidy level is the number of sets of chromosomes in a cell. Meiosis lowers the ploidy level. That halving is the point: a gamete is an egg or sperm cell, and correctly formed gametes contain a haploid set of chromosomes, so that when two of them meet the new cell starts with two sets again, not four.

The cast: homologs and sister chromatids

Two partnerships must not be confused. Homologous chromosomes are matched pairs of chromosomes that carry the same genes in the same places along their length. Sister chromatids are the two identical copies made when a chromosome is replicated, held together at the centromere by cohesin proteins. Homologs are partners; sisters are photocopies. Meiosis I separates the partners, and meiosis II separates the photocopies. Keep those two sentences apart and the rest of this topic follows.

The picture shows a cell with 2n = 4 at metaphase I: the homologous pairs sit on the equator as two tetrads. A tetrad is a synapsed pair of homologous chromosomes; because each homolog is two sister chromatids, the pair has four chromatids.

Meiosis I: the reduction division

Meiosis I is a reduction division: a nuclear division that gives each daughter nucleus half as many chromosome sets as the parent nucleus. In prophase I, synapsis occurs: synapsis is the tight pairing of homologous chromosomes in prophase I. While the homologs are paired, crossing over occurs in prophase I: crossing over is the exchange of chromosomal segments between non-sister chromatids of homologous chromosomes. The swap produces a recombinant chromatid, a chromatid carrying a combination of maternal and paternal alleles that did not exist before the crossover. In metaphase I, independent assortment occurs: the homologous pairs arrange themselves at the metaphase plate, and which partner faces which pole is decided pair by pair. In anaphase I, homologous chromosomes separate; this is the step that halves the set. Meiosis I includes cytokinesis, and meiosis I results in two haploid cells. Each product of meiosis I receives one homolog of every homologous pair.

Meiosis II: the mitosis-like division

Meiosis II is like mitosis in its mechanics, and it is not a reduction division: the set is already halved, and this division splits what remains. In anaphase II, sister chromatids separate; this division splits the copies that remain. Meiosis II includes cytokinesis, and meiosis II results in four haploid cells. Meiosis produces four nuclei; mitosis does not. The four products are not identical copies of the parent: meiosis does not produce identical nuclei. The two cells from meiosis I received different homologs, and crossing over has made even sister chromatids different, so the products usually differ from one another in their combinations of alleles.

Meiosis I, stage by stage

In prophase I, homologous chromosomes pair up and condense; synapsis occurs and then chiasmata may form. A chiasma is the visible X-shaped point where non-sister chromatids have crossed over. The meiotic spindle begins to form, centrosomes move to opposite poles of the cell, and the nuclear envelope breaks down.

In metaphase I, meiotic spindle fibers align homologous pairs of chromosomes along the equator of the cell at the metaphase plate. In anaphase I, homologous chromosomes separate, while sister chromatids remain attached, as meiotic spindle fibers pull chromosomes toward poles.

In telophase I, the meiotic spindle breaks down, a new nuclear envelope develops, a cleavage furrow (animal cell) or cell plate (plant cell) forms, and cytokinesis occurs. Two haploid daughter cells are formed at the end of meiosis I.

Meiosis II, stage by stage

In prophase II, the meiotic spindle forms; sister chromatids connected at the centromere attach to the meiotic spindle. In metaphase II, chromosomes align along the metaphase plate; the kinetochore of each chromatid is attached to a microtubule extending from the poles.

In anaphase II, proteins at the centromeres break down, and sister chromatids are pulled apart and toward opposite poles in the cell. In telophase II, the meiotic spindle breaks down, a new nuclear envelope develops, a cleavage furrow (animal cell) or cell plate (plant cell) forms, chromatids begin to decondense, and cytokinesis occurs. Four haploid daughter cells are formed, each holding n chromosomes, and each of those chromosomes is an unduplicated (single-chromatid) chromosome.

Meiosis beside mitosis, and why the halving matters

The two divisions share their machinery: mitosis uses a spindle to move chromosomes, and meiosis uses a spindle too. Their outcomes are opposites. Mitosis does not lower the ploidy level; meiosis does. Mitosis produces identical nuclei; meiosis does not. Synapsis is part of meiosis; mitosis does not include synapsis, because a mitotic cell never pairs its homologs. Crossing over occurs in meiosis I; mitosis does not include crossing over, and neither does meiosis II. Keep the comparison in that shape — same spindle, different partnerships — and the two processes stop blurring.

The halving exists for the meeting that follows. Fertilization is the union of two haploid gametes, which restores the diploid condition. Meiosis prevents the chromosome number from doubling in every generation: without a halving division first, every generation would double its chromosome number, and with it the number holds steady while the contents are reshuffled. How that reshuffling creates diversity is the next topic.

Warning.

Three traps to leave behind. Counting chromosomes, count joined sister chromatids as one chromosome until they separate. Halving happens in meiosis I, not meiosis II: by the time the second division starts, the cells are already haploid. Each final cell carries one set; in animals, the products of meiosis can become gametes, and fertilization restores two.

Adapted from Biology by OpenStax (Rice University), licensed under CC BY 4.0. Access for free at https://openstax.org/details/books/biology

Worked examples

Each example is the kind of question the AP exam asks on this topic, worked the way a teacher would talk it through.

Example.

1. Counting through the divisions. A species has 2n = 4. How many chromosomes and how many DNA molecules does one cell hold at metaphase I, and what has changed by the end of meiosis?

The counts for metaphase I: 4 chromosomes and 8 DNA molecules, in two homologous pairs (tetrads). Every chromosome there is two sister chromatids, the two copies made when the chromosome was replicated — which is why the molecule count is twice the chromosome count.
Meiosis I then separates the homologs and results in two haploid cells. The halving happened there: one partner of each pair went to each cell.
Meiosis II separates the sister chromatids and results in four haploid cells, each holding one set — in this species, 2 chromosomes, each a single chromatid.

Result.

Metaphase I: 4 chromosomes, 8 DNA molecules. At the end: four haploid cells of 2 chromosomes each.

Example.

2. Two divisions, two different partings. A student says: "Meiosis halves the chromosome number twice, once in each division." Correct the sentence.

Meiosis I is the reduction division: it separates homologous chromosomes, giving each daughter nucleus half as many chromosome sets as the parent.
Meiosis II separates sister chromatids. It is like mitosis and is not a reduction division, because the cells entering it are already haploid.
So the number is halved once, in the first division; the second division only unpacks the copies.

Result.

Halved once, in meiosis I; meiosis II separates sisters in already-haploid cells.

Example.

3. Why the four products differ. Four haploid products come from one parent cell. A student expects them to be identical, like the products of mitosis. Why are they not?

Mitosis produces identical nuclei. Meiosis does not produce identical nuclei.
Before the separations, crossing over in prophase I exchanged segments between non-sister chromatids, making recombinant chromatids with new combinations of alleles.
And at metaphase I the homologous pairs assorted independently, so which parental partner each gamete receives was decided pair by pair, by chance.

Result.

The two cells from meiosis I differ because each receives a different homolog of every pair (separation of homologs in anaphase I); independent assortment of the pairs at metaphase I means which combination of maternal and paternal homologs goes to each cell varies by chance; crossing over makes even the two cells from the same half differ. Together they make the products usually differ from one another in their combinations of alleles.

Practice

Answer AP-style questions on meiosis. Each answer is checked, with the full solution, and every try brings new numbers. Practice needs a free account: it keeps your progress.